The present invention relates generally to methods of conducting paired synthesis reactions electrochemically, and more specifically, to the preparation of ethylene glycol at the cathode of an electrochemical cell while simultaneously producing a regeneratable redox reagent at the anode of the same cell, which redox reagent can be reacted with an organic substrate to prepare a secondary product indirectly.
Ethylene glycol is a major industrial chemical with worldwide production of about 20 billion pounds per year. Ethylene glycol is widely used in manufacturing polyester films and fibers and as an automotive coolant and antifreeze. The major source of ethylene glycol is from epoxidation of ethylene which is derived from petroleum, followed by hydration to form the glycol. However, dwindling petroleum reserves and petroleum feedstocks coupled with escalating prices has led to development of alternative routes based on syngas. Representative processes are described in U.S. Pat. Nos. 3,952,039 and 3,957,857. In a recent patent to N. L. Weinberg, U.S. Pat. No. 4,478,694, an electrochemical route is described wherein formaldehyde is electrohydrodimerized at the cathode to produce ethylene glycol at high current efficiencies and yields according to the equation: EQU 2CH.sub.2 O+2H.sup.+ +2e.sup.- .fwdarw.HOCH.sub.2 CH.sub.2 OH (I)
Heretofore, many electrochemical methods of manufacturing organics, including synthesis of ethylene glycol were not widely accepted mainly because they were generally viewed as being economically unattractive. Significant effort has been made to improve the economics for the electrochemical synthesis of ethylene glycol. One such example is found in U.S. Pat. No. 4,478,694 which includes conducting the reaction while also performing a "useful anode process." The expression "useful anode process" was coined to denote reactions occurring at the anode for lowering power consumption or forming in-situ a product which can be utilized in the synthesis of ethylene glycol. Specifically, U.S. Pat. No. 4,478,694 discloses the oxidation of hydrogen gas at the anode for purposes of forming protons used in formaldehyde electrohydrodimerization at the cathode according to equation (I) above. U.S. Pat. No. 4,478,694 also discloses as a useful anode process the anodic oxidation of methanol to formaldehyde which in-turn is used as a catholyte feedstock in the electro-reduction reaction.
U.S. Pat. No. 4,478,694, however, fails to disclose electrochemical synthesis reactions in which secondary products formed at the anode are not used in the synthesis of ethylene glycol at the cathode. That is, the U.S. patent does not teach or suggest the preparation of secondary products formed by reacting "indirectly", generated anode products with ethylene glycol synthesized at the cathode to produce a third product, e.g. dimers, trimers, tetramers or other polymers. Terms like "indirect" or "indirectly" referring to electrolysis product(s), as used herein are intended to mean organic products which are not formed directly at the anode by oxidation of an organic feed, but instead are produced by reaction of the organic feed with a regeneratable redox reagent, as a consequence of the latter's oxidation at the anode.
Accordingly, the present invention contemplates even more economically attractive electrochemical synthesis reactions with the simultaneous production of ethylene glycol wherein two or more useful products are generated simultaneously at the anode and cathode of the same electrochemical cell, and where the anode product(s) are formed indirectly, hereinafter referred to as "paired electrochemical synthesis". The process is specially significant in light of the paired products ability to share in capital costs for cells, as well as operating costs, and particularly power.
But, the process is also quite surprising in view of the fact that usually paired reactions cannot be conducted successfully side-by-side in the same electrochemical cell due to fundamental incompatibilities in cathodic and anodic reactions, e.g. operating conditions and cell components, to name but a few. More specifically, in the paired electrochemical synthesis of ethylene glycol at the cathode while simultaneously producing a regeneratable redox reagent at the anode for reaction with an organic substrate to form a secondary product indirectly, many of the more preferred metal ions of redox couples, such as Ce.sup.+3 or Ce.sup.+4 ; Cr.sup.+3 and Co.sup.+2 or Co.sup.+3 could pass from the anolyte compartment through the membrane separator to the catholyte compartment in competition with protons which are required for the cathodic process in accordance with equation (I) above. In the absence of sufficient protons a pH imbalance occurs on the cathode side. This will depress the conversion efficiency of formaldehyde to ethylene glycol which translates into greater power consumption and costs per unit of product produced. In addition, passage of these metal ions of regeneratable redox reagents from the anode to the cathode side, has a tendency to inhibit the electroreduction of formaldehyde to ethylene glycol by "poisoning" the carbon cathode. Consequently, the hydrogen current efficiency increases and the desired ethylene glycol current efficiency of at least 70 percent decreases. Passage of metal redox reagent ions from the anolyte to the catholyte compartment also means losses of valuable redox metal salts, necessitating increased costs for their makeup, recovery and/or disposal.
In addition to the foregoing problems associated with paired electrochemical synthesis with simultaneous production of ethylene glycol, certain regeneratable redox reagents have a tendency to precipitate in membrane/separators leading to increased IR loses and membrane destruction. Membranes are also subject to destruction by oxidants formed in the anolyte. Moreover, back-transfer of catholyte species, particularly organics, such as formaldehyde, ethylene glycol and oxidizable electrolyte anions, such as formate, into the anolyte causes deactivation of oxidant species and current efficiency losses. Accordingly, the present invention provides for important technical improvements in the electrochemical production of ethylene glycol making this method even more economic through a paired reaction format.